Create a new Shell Mode accessible through shift+tab

### Describe the feature or problem you'd like to solve

I want to run terminal (shell) commands by default and only query the agent when needed.

### Proposed solution

Create a new mode, Shell Mode, which runs terminal commands by default. Then I can use `!` or `/agent` to access the agent when I need to.

### Example prompts or workflows

If I try and run `ls` or `git status` it queries the agent wasting time and costing credits.

I would like to be able to:
1. Press `shift+tab` until I enter "Shell Mode"
2. Shell mode assumes my text is a shell command by default
3. To access agent features, I use `!` (opposite of the default mode) or `/agent`
4. To return to normal mode, I can use `shift+tab` to toggle back to it.

This way the user has the ability to use copilot-cli as a shell first and an agent second rather than the other way around.

BONUS: Have copilot-cli automatically detect shell commands vs. agent queries and smartly choose the correct path

### Additional context

_No response_

Das Issue nennt keine Dateien, Tests oder Einstiegspunkte; beginne damit, den bestehenden Shift+Tab-Moduswechsel-Flow in copilot-cli zu finden. Definiere den Abschluss als einen Shell Mode, der standardmäßig Shell-Befehle ausführt, ! oder /agent für den Agentenzugriff unterstützt und mit Shift+Tab zurückwechselt.

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